A conventional large-capacity ozone generator is typically configured as shown in FIG. 10 to construct a several-tens-to-several-hundreds-of-kilograms-per-hour class, large-capacity ozone generator. This large-capacity ozone generator is adopted to use ozone for advanced sewage treatment and pulp bleaching.
FIG. 10 shows an example of a conventional, large-capacity ozone generator that generates ozone at a rate of 35 kg/h. The conventional large-capacity ozone generator 1100 includes a water tank for cooling, a huge tank (3000 mm in diameter and 4500 mm in length) filled with an oxygen material gas, and 600 cylindrical glass high-voltage pipes 5, which are 40 mm in diameter and 2000 mm in length. A cylindrical high-voltage electrode pipe is mounted in the cylindrical glass high-voltage pipes. The cylindrical high-voltage electrode pipe is coated with a high-voltage electrode 3.
The reference numeral 2000 denotes a transformer for supplying a high voltage to the plurality of cylindrical glass high-voltage pipes 5. The reference numeral 3000 denotes a plurality of series-connected reactors for suppressing an inverter output current. The reference numeral 4000 denotes a single-phase inverter element for outputting a single-phase AC voltage from a DC voltage input. The reference numeral 5000 denotes a converter (rectifier) for supplying a DC voltage that is to be input into an inverter. The reference numeral 6000 is an input transformer for cutting off a third harmonic, which arises out of a load on a three-phase commercial AC power supply and ozone generator. The reference numeral 4100 denotes a drive circuit for driving the inverter element 4000. The reference numeral 4200 denotes a control circuit for the inverter element 4000. The reference numeral 4300 denotes a computer for exercising management and issuing condition setup instructions concerning, for instance, the status of the ozone generator and the current output to the inverter. FIG. 11 schematically indicates the number of 40 mm diameter cylindrical glass pipes 5 that can be mounted within a cross section 200 mm square. FIG. 12 is a characteristics diagram illustrating the relationship between an inverter instruction signal and inverter output waveform.
In the conventional large-capacity ozone generator, a three-phase commercial AC power supply enters the input transformer 6000. A converter 5000 converts the output of the input transformer 6000 to a DC voltage. The obtained DC voltage enters the inverter element 4000. The inverter element 4000 converts the DC voltage to an AC voltage having a frequency of approximately 1 kHz.
The inverter element 4000 receives ozone performance conditions from the computer 4300 and forwards them to the control circuit 4200. The control circuit 4200 creates an inverter control signal. Predetermined control signals are alternately input from the drive circuit 4100 to the inverter element 4000 as the ON-OFF signals for two instruction signals 7001a, 7001b shown in FIG. 12. The inverter then outputs an AC rectangular voltage that is synchronized with the resulting pulse.
When the AC rectangular voltage enters the transformer 2000 via the series-connected reactors 3000, which suppress the inverter output current, the secondary side of the transformer outputs a high-voltage AC waveform Y1, which has a voltage of approximately 10 kV and is similar to a 1 kHz sine waveform. This high-voltage AC waveform is applied to the plurality of cylindrical glass high-voltage pipes 5 from a high-voltage terminal 120 of the large-capacity ozone generator 1100. Another low voltage of a secondary voltage is connected to a ground terminal (low-voltage terminal) YG of the large-capacity ozone generator 1100. When a high voltage is applied to the ozone generator 1100 as described above, a silent electric discharge occurs between the high-voltage electrode 3 and low-voltage electrode via the cylindrical glass high-voltage pipes 5, which are dielectrics. Consequently, ozone is generated so that an ozonized gas can be obtained from an ozone outlet 110 of the ozone generator 1100.
The performance specifications for the above conventional ozone generator 1100 are as indicated in Table 1 if it is a 35 kg/h class, large-capacity ozone generator. The large-capacity ozone generator is a huge apparatus, which has a diameter of 3000 mm, a length of 4000 mm, and a cubic capacity of 28 m3. Further, the power supply capacity is 452 kW. The load power factor is approximately 30%. Therefore, the load current is 150 A. The load voltage is 10 kV. The load capacity is 1500 kVA. It means that the transformer 2000 is extremely large.
TABLE 1Conventional cylindrical multi-pipe method specificationsDischarge cell cubic volume20 cm × 20 cm × 20 cm(0.003 m3)Number of discharges600 (φ40-2000)Discharge area (m2)150.72Ozone generator tank cubicφ3000-4000 mm (28 m3)capacityRated ozone concentration(g/m3)180Maximum ozone concentration (g/m3)220Ozone yield (kWh/kg−03)11Discharge power density (W/cm2)0.3Discharge power (kW)452Ozone generation rate (kg/h)41.11Gas flow rate (L/min)3806.1Operating frequency (kHz)1Load power factor (%)30Load current (A)150.72Load voltage (kV)10
The relationship between the discharge load resistance and the ozone capacity of the ozone generator is as indicated in a characteristics diagram in FIG. 13. In FIG. 13, characteristic E represents the characteristic of the cylindrical multi-pipe ozone generator that operates on a conventional AC voltage having a frequency of 1 kHz. The discharge resistance characteristic is as indicated by 8001a. The larger the capacity is, the smaller the discharge resistance (load impedance) becomes. Therefore, it is difficult to disperse a silent electric discharge over a large discharge area uniformly and steadily.
The relationship between the frequency and the discharge voltage to be applied to the ozone generator is as indicated in a characteristic diagram in FIG. 14. In FIG. 14, characteristic F (solid line) is a discharge voltage characteristic prevailing at a discharge gap length of 0.1 mm and at a discharge power density of 1.5 W/cm2. Characteristic G (dotted line) is a discharge voltage characteristic prevailing at a discharge gap length of 0.3 mm and at a discharge power density of 0.3 W/cm2. The lower the frequency is, the higher the discharge voltage becomes. The higher the discharge power density, the higher the discharge voltage. The conventional cylindrical multi-pipe ozone generator generates ozone at a discharge gap length setting of approximately 0.3 to 0.6 mm, at a frequency setting of 1 kHz to 3 kHz, and at a discharge power density setting of 0.3 W/cm2. Therefore, the operation region in FIG. 14 is 8001b and the discharge voltage is approximately 10 kV. If the discharge power density is higher than 0.3 W/cm2, the discharge voltage increases by more than 12 kV. Therefore, the discharge power density should not be higher than 0.3 W/cm2 from the viewpoint of practical apparatus use.
Further, when a plurality of 40 mm diameter and 2000 mm long cylindrical glass high-voltage pipes 5 are inserted at an operating frequency of 1 kHz and at a discharge power density of 0.3 W/cm2, the cubic capacities of a conventional ozone generator generating ozone at a rate of 7 kg/h and a conventional ozone generator generating ozone at a rate of 70 kg/h are as large as 0.7 m3 and 56 m3, respectively, as indicated by 8003a in FIGS. 15 and 8003b in FIG. 16. Since the power factor of the ozone generator is as low as 30% at an operating frequency of 1 kHz, the discharge capacity is extremely large. Thus, the transformer 2000 and inverter element 4000 are very large. Consequently, the large-capacity ozone generator is a huge system.
Various prior arts were disclosed to technically improve the conventional large-capacity ozone generator.
First of all, a large-capacity ozone generator, which was disclosed by Japanese Patent Application No. 2002-306941 prior to the present invention, includes a chamber. The chamber houses a plurality of discharge cells. The discharge cells have a discharge space having a gap of approximately 0.1 mm. The discharge space is provided between two rectangular flat-plate electrodes and two flat-plate electrodes via a dielectric. In this ozone generator, a plurality of discharge cell electrodes, which are mounted in the chamber, are connected in parallel. A high AC voltage is applied between the electrodes so that a uniform dielectric barrier discharge (silent electric discharge) occurs in the gap section of each discharge cell via the dielectric. At the same time, a material gas based on an oxygen gas is allowed to enter the chamber so that a gas uniformly passes from the outer circumference of a discharge cell to the discharge space having a gap as short as 0.1 mm. Consequently, a large amount of ozone gas is obtained. Further, the operating frequency is set to 10 kHz to reduce the discharge voltage. In addition, a discharge power density of 1.0 W/cm2 is employed for performance improvement.
A silent electric discharge type ozone generator power supply, which was disclosed by Japanese Patent Laid-Open No. 1997-59006 prior to the present invention, includes a plurality of discharge pipes and a transformer. In the transformer, the discharge pipes are arranged and three-phase-connected to a three-phase current type inverter in order to improve the load power factor and ozone generation efficiency.
An ozone generation structure, which was disclosed by Japanese Patents Laid-Open No. 1994-305706 and Laid-Open No. 1995-240268 prior to the present invention, includes an n-phase AC power supply, which generates an n-phase AC output, and n discharge electrode rods, which are positioned in a discharge chamber. One end of each discharge electrode rod is positioned to form an n-sided equilateral polygon. An n-phase AC voltage is applied to the n discharge electrode rods so that a corona discharge occurs near the electrode rods between electrodes. Ozone is then generated by allowing a material gas containing oxygen to flow along the discharge electrode rods, which are arranged to form an n-sided equilateral prism. This polyphase AC multi-electrode corona discharge device exhibits higher efficiency than a single-phase corona discharge device because the former can use a planar discharge between electrodes for ozone generation purposes.
Another ozone generation structure, which was disclosed by Japanese Patent Publication No. 1996-22724 prior to the present invention, includes an n-phase AC output device for generating an n-phase AC output and n discharge electrode rods that are positioned in a discharge chamber. One end of each discharge electrode rod is positioned to form an n-sided equilateral polygon. The other end of each discharge electrode rod is positioned close to a vertex, that is, positioned to form an n-sided equilateral pyramid. An n-phase AC voltage is applied to n discharge electrode rods so that a corona discharge occurs near the electrode rods between electrodes. Ozone is then generated by allowing a material gas containing oxygen to flow from the base section of the discharge electrode rods, which are positioned to form an n-sided equilateral pyramid, to the vertex of the n-sided equilateral pyramid. This polyphase AC multi-electrode corona discharge device exhibits higher efficiency than a single-phase corona discharge device because the former can use a planar discharge between electrodes for ozone generation purposes. This invention is equivalent to the invention disclosed by Japanese Patent Laid-Open No. 1995-240268 although the two inventions differ in electrode rod arrangement.
Another ozone generator, which was disclosed by Japanese Patent Laid-Open No. 1995-157302 prior to the present invention, includes transformers to which a three-phase AC power supply is star-connected (Δ-connected). One end of a secondary terminal of each transformer is connected to a cylindrical terminal as a common electrode. The other end of the secondary terminal is connected to a rod-shaped high-voltage electrode. A high AC voltage is applied to the rod-shaped high-voltage electrode.
In another ozone generator, which was disclosed by Japanese Patent Laid-Open No. 1998-25104 prior to the present invention, discharge cells, which include a plurality of dielectrics and high-voltage electrodes, are multi-layered over a single ground electrode (low-voltage electrode surface). Ozone power supplies, which are based on the combination of a three-phase inverter and a three-phase transformer, and multi-layered discharge cells are divided into three cell groups. A three-phase high AC voltage is applied to the high-voltage electrode section of each cell group. In an alternative embodiment, four ozone power supplies are connected to supply a plurality of high AC voltages to a single discharge chamber. The use of a 3-phase ozone power supply makes it possible to drive three discharge cells and make the power supply section compact and inexpensive.
In another ozone generator, which was disclosed by Japanese Patent Laid-Open No. 2001-26405 prior to the present invention, ozone generator units are stacked. The resulting ozone generator is compact although it has a large capacity.
An ultracompact ozone generator unit 100, which is shown in FIG. 7, was disclosed by Japanese Patent Application No. 2002-306941 prior to the present invention. In FIG. 7, the reference numeral 3 denotes a high-voltage electrode, which is very thin (approximately less than 100 μm in thickness) and sandwiched between thin flat-plate dielectrics 5, which have a thickness of less than 1 mm. The reference numeral 7 denotes a flat-plate electrode, which is a rectangular low-voltage electrode that is 20 mm in width, 500 mm in length, and several millimeters in thickness. A plurality of very thin high-voltage electrodes 3 that are sandwiched between flat-plate dielectrics 5, which are less than 1 mm in thickness, and a plurality of rectangular low-voltage electrodes 7, which are several millimeters in thickness, are stacked alternately to form an ozone generator unit 100, which measures approximately 200 mm by 200 mm and has a length of 500 mm. Two etched flat plates are joined together so that the interior of each rectangular low-voltage electrode 7 is structured to provide a water cooling path and ozone gas acquisition scheme. To form a short gap discharge space, a 0.1 mm protruding spacer is provided on both sides of each rectangular low-voltage electrode 7. To invoke a uniform oxygen gas flow in the rectangular discharge space and improve the ozone generation performance, the employed electrode is devised so as to obtain a plurality of ozone gases from the rectangular discharge space. The water inlet and outlet for electrode cooling and the ozone gas outlet are not shown in FIG. 7. However, the detailed configuration is indicated in Japanese Patent Application NO. 2002-306941.
Japanese Patent Application No. 2002-306941 states that an ultracompact ozone generator 1100, whose capacity is one several-th of the conventional capacity, can be configured, as indicated by 8004a in FIG. 15 or by 8004b in FIG. 16, by mounting a plurality of discharge units 100 in a single discharge chamber as indicated in FIG. 6, raising the power frequency from a conventional level of 3 kHz or lower to approximately 6 to 20 kHz as indicated by 8002b in FIG. 14, and increasing the discharge power density of the ozone generator from a conventional level of 0.3 W/cm2 to approximately 1 to 2 W/cm2.
In the conventional cylindrical multi-pipe ozone generator, the ozone performance characteristic is such that the maximum ozone concentration is approximately 220 g/m3 as indicated by 8005a in FIG. 17. In the multi-layer flat-plate ozone generator, on the other hand, the maximum ozone concentration is 340 g/m3 as indicated by 8005b in FIG. 17. It means that the resulting ozone concentration is 1.5 times higher than the conventional level. As regards the ozone concentration achieved by the large-capacity ozone generator, the electrical energy required for acquiring ozone gas at a rate of 1 kg/h (ozone yield) is more important than the maximum ozone concentration. If greater importance is attached to the ozone yield as indicated in FIG. 17, the ozone concentration is approximately 180 g/m3 during the use of the cylindrical multi-pipe ozone generator and 210 g/m3 during the use of the multi-layer flat-plate ozone generator. It means that the ozone yield is increased by less than 20%.
Table 2 shows the comparison between the conventional cylindrical multi-pipe ozone generator shown in FIG. 11 and the multi-layer flat-plate ozone generator (on which the present invention is based) shown in FIG. 7. The table indicates that the multi-layer flat-plate ozone generator is superior to the conventional cylindrical multi-pipe ozone generator in terms of capacity and compactness. However, the conventional cylindrical multi-pipe ozone generator is superior to the multi-layer flat-plate ozone generator in terms of apparatus simplicity, assembling ease, and production cost. However, Japanese Patent Application No. 2002-306941 states that a rectangular, high-performance, multi-layer flat-plate ozone generator can provide apparatus simplicity and assembling ease. It means that the multi-layer flat-plate ozone generator is also becoming advantageous in terms of capacity.
TABLE 2Cylindrical multi-Multi-layer flat-pipe typeplate typeDischarge cell20 cm × 20 cm × 50 cm20 cm × 20 cm × 50 cmcubic volumeNumber of25 (φ40-500)32 (200 × 5-500)dischargesDischarge1.512.76area (m2)Rated ozone180210concentration(g/m3)Maximum ozone220340concentration(g/m3)Ozone yield119.5(kWh/kg−03)Discharge power0.31.1density (W/cm2)Discharge power4.5230.41(kW)Ozone generation0.413.20rate (kg/h)Gas flow rate38.1254.1(L/min)Operating110frequency(kHz)Voltage (kV)106
In an ozone generator including a short gap discharge device, in which the frequency setting is several kilohertz or higher and the discharge power density is 0.5 W/cm2 or higher, the relationship between the ozone capacity and discharge resistance, which is shown in FIG. 13, indicates that the maximum single unit capacity is limited. In FIG. 13, discharge characteristics A to D at a discharge power density of 1.4 W/cm2 represent the characteristics of the multi-layer flat-plate ozone generator. Discharge characteristic E represents the characteristic of the conventional cylindrical multi-pipe ozone generator and prevails at a frequency of 1 kHz and at a discharge power density of 0.3 W/cm2. As indicated by characteristic region 8002a for the multi-layer flat-plate type, the use of a large-capacity ozone generator remarkably reduces the discharge resistance of the ozone generator and makes it difficult to disperse a discharge over the entire surface and generate stable ozone. As a result, it is found that the use of a large-capacity ozone generator in place of a small-capacity ozone generator cannot obtain high-concentration ozone and reduces the ozone yield. Further, the results of experiments indicate that a discharge resistance of approximately 20 ohms or more is required for increasing the ozone generator capacity in consideration of apparatus controllability and stability. It is therefore found that the maximum single unit capacity of the multi-layer flat-plate ozone generator is approximately 10 kg/h. An increase in the single unit capacity results in a poor discharge power factor and increases the peak current for the load current value. It means that an economical limit is imposed due, for instance, to an increase in the power consumption for ozone generation. Consequently, the single unit capacity of the ozone generator is limited. Furthermore, an increase in the single unit capacity remarkably reduces the discharge cell load impedance. It means that the power supply control capability for stable supply of generated ozone is extremely degraded. As a result, various problems, including the inability to increase the single unit capacity, have arisen.
Meanwhile, an ozone water treatment apparatus, which uses ozone, or an ozone pulp bleaching apparatus, which uses ozone for pulp bleaching, requires several tens to several hundreds of kilograms of ozone per hour. If a several-hundreds-of-kilograms-per-hour class, cylindrical, multi-pipe ozone generator is employed, a large-scale facility having a cubic volume of approximately 200 m3 results due to electrode shape. In the field of water treatment or pulp bleaching, which requires the use of ozone, therefore, an extremely compact apparatus having a single unit capacity as large as several tens of kilograms to 100 kilograms per hour and exhibiting high ozone yield and high ozone concentration is demanded.
To configure an ozone generator having a single unit capacity as large as several tens of kilograms to 100 kilograms per hour, it is found necessary, as described above, not only to employ a chamber configuration that accepts a plurality of discharge cells, but also to review the ozone generator system configuration including an ozone power supply. It is also found necessary that a plurality of ozone power supplies be arranged in an ozone generator unit and rendered electrically independent of each other to control the amount of ozone generation.
Patent Document 1: Japanese Patent Laid-Open No. 1997-59006
Patent Document 2: Japanese Patent Laid-Open No. 1994-305706
Patent Document 3: Japanese Patent Laid-Open No. 1995-240268
Patent Document 4: Japanese Patent Publication No. 1996-22724
Patent Document 5: Japanese Patent Laid-Open No. 1995-157302
Patent Document 6: Japanese Patent Laid-Open No. 1998-25104
Patent Document 7: Japanese Patent Laid-Open No. 2001-26405